It is well known that, in systems using imaging devices such as television cameras, telecine cameras and the like, the visual image characteristics of the scene being viewed are at times incorrectly reproduced. For example, the use of such imaging devices frequently introduces distortion or deviation into such visual image characteristics as colour, brightness, tonal scale, geometric linearity, sharpness and position. In the same way photographic film techniques similarly cause distortion in the recorded images.
In an attempt to alleviate these problems, it is customary to incorporate adjustment means in imaging devices such as television cameras to allow visual image characteristic adjustment. For the same reason, it is well known to provide photographic cameras with similar adjustment means, such as filters, whereby the colour balance, brightness and/or other visual image characteristic of the scene being photographed can be adjusted.
In the majority of cases, these adjustments are intended to produce an image which is a reproduction of the scene which is as faithful as possible under a specific illumination. However, this is not the only reason for making such adjustments. In some cases, it is desired to deliberately change the colour balance, brightness and/or some other visual image characteristic of the image of the scene, for artistic or other reasons.
Many different factors affect the quality of the images produced by such imaging devices or cameras. These factors include the spectral characteristics of the illuminating light source, the absorption spectrum of the scene, and the transmission characteristics of the glassware, i.e., the lens, splitting blocks, etc., the cross-over wave lengths of the dichroic image dividers, the transmission wave bands of the trim filters, and the characteristics of the image sensing components of the tele-cine or video camera itself. Additionally, the optical systems of such devices can introduce geometric distortion.
For many years, the method used to evaluate the signals produced by imaging devices such as television cameras has been to aim the camera at a test target comprising one or more patterns representing a range of values for the particular visual image characteristic. The electrical signals produced by the camera, when aimed at such a target, were then evaluated. This evaluation was either objective, i.e., the signals were observed and measured on instruments such as wave-form monitors and vectorscopes. Alternatively, the image represented by such signals was evaluated subjectively. For example, by passing such signals to an accurately aligned colour monitor, the resulting image of the test target could be viewed and subjectively evaluated.
Having so evaluated the image signal of the test target, any necessary corrections and adjustments could then theoretically be made by passing the signal through an image signal modifier adapted to adjust the selected visual image characteristic of the image signal. The same adjustment would then be made to the image signals provided by the imaging device or camera while actual scenes were being shot. Such image signal modifiers are known in the industry, and incorporate controls for adjusting the colour balance, brightness and/or other visual image characteristics of a signal, prior to dissemination, whether by broadcasting over the air waves, or through a closed circuit or the like. Such modifiers were used to adjust and/or correct the selected visual image characteristic(s) of the image signals.
While these known systems produced considerable improvements in the quality of the image disseminated, or broadcast, and produced on a screen or monitor, problems still arose.
For example, if such an image signal is evaluated in a device such as a wave-form monitor or vectorscope, it is well known that such equipment is less sensitive than the human eye, and will overlook subtle differences which would be readily apparent to the human eye. Such images are subjective by nature, and can be seriously impaired by relatively minor errors. For example, a difference of ten IRE units, as measured on a wave-form monitor, in the blue channel, will not necessarily appear as the same difference in brightness as a difference of ten IRE units on the read channel, when both are viewed on a colour monitor.
The human eye can, however, immediately detect such a difference.
On the other hand, where the image produced by the imaging device from the test target is displaced on a colour monitor for subjective evaluation, it is necessary for an observe to compare the image displayed on the colour monitor with the actual test target itself. These may be at a distance from one another, viewed under different lighting conditions, or of different colour temperatures. These factors may make it difficult to make a judgement concerning any adjustment that may be required.
A further disadvantage of earlier systems was the fact that, even when the image of the test target was reproduced on a monitor, the test image, even when the camera has been adjusted, would still not necessarily reproduce the various colours in a theorectically optimized manner. Imperfections arose, whether produced by imperfections in the colour monitor, or by the manner in which the target was illuminated, or by an imperfect imaging device.
These imperfections were, in turn, carried over, in the case of, for example, a television broadcast, in the image signals broadcast by the system, and were then reproduced on monitors or television sets in homes.
In the case of photographic film cameras, it is customary to simply photograph a scene using the camera with a selected filter attached. After the film has been exposed and developed, and prior to or during dissemination of the film, e.g., by screening for television, the image of the film could be evaluated subjectively by observing the photographic reproduction of the scene. If necessary, the filter could then be changed, or other adjustments made in an attempt to obtain a more faithful reproduction.
In would clearly be desirable, if were possible to evaluate image signals in a more accurate manner and, if desired, to modify or adjust one or more visual image characteristics of such signals so that such signals as subsequently disseminated or broadcast would conform more closely to a faithful reproduction of the scene being viewed. In this way, imperfections in the image as reproduced and seen would be reduced; the image as actually seen, for example, on a monitor or television set would be as close to perfect as was possible.
It would also be desirable if it were similarly possible by the use of a visual image modifier to modify visual image characteristics of such image signals so as deliberately to distort one or more characteristics of the image, for artistic or other effects. It could be desirable, for example, to do this after the scene had been photographed or recorded by the imaging device by passing the signal through a image signal adjusting device. The invention is not, therefore, restricted solely to evaluating and possibly correcting for errors or deficiencies in visual image characteristics but also encompasses deliberately distorting such visual image characteristics where desired.
The term 37 modifying" as used herein is intended to encompass both correcting and distorting of one or more visual image characteristics relative to a predetermined value for such characteristic.